SixtyPical

SixtyPical is a very low-level programming language, similar to 6502 assembly, with block structure and static analysis through abstract interpretation.

It is a work in progress, currently at the proof-of-concept stage.

It is expected that a common use case for SixtyPical would be retroprogramming for the Commodore 64, VIC-20, Apple ][, etc.

Many SixtyPical instructions map precisely to 6502 opcodes. However, SixtyPical is not an assembly language. The programmer does not have total control over the layout of code and data in memory. The language has a type system which distinguishes addresses from non-addresses (16-bit values for which it does not make sense to treat them as addresses.) Some 6502 opcodes have no SixtyPical equivalent. Some SixtyPical instructions are named after 6502 opcodes, but generate slightly different (safer, but intuitively related) sequences of opcodes. Et cetera.

sixtypical is the reference implementation of SixtyPical. It is written in Haskell. It can currently parse and analyze a SixtyPical program, and will eventually be able to compile it to an Ophis assembler listing.

Concepts

Routines

Instead of the assembly-language subroutine, SixtyPical provides the routine as the abstraction for a reusable sequence of code.

A routine may be called, or may be included inline, by another routine.

There is one top-level routine called main which represents the entire program.

The instructions of a routine are analyzed using abstract interpretation. One thing we specifically do is determine which registers and memory locations are not affected by the routine.

If a register is not affected by a routine, then a caller of that routine may assume that the value in that register is retained.

Of course, a routine may intentionally affect a register or memory location, as an output. It must declare this. We're not there yet.

Addresses

The body of a routine may not refer to an address literally. It must use a symbol that was declared previously.

An address may be declared with reserve, which is like .data or .bss in an assembler. This is an address into the program's data. It is global to all routines.

An address may be declared with locate, which is like .alias in an assembler, with the understanding that the value will be treated "like an address." This is generally an address into the operating system or hardware (e.g. kernal routine, I/O port, etc.)

Inside a routine, an address may be declared with temporary. This is like static in C, except the value at that address is not guaranteed to be retained between invokations of the routine. Such addresses may only be used within the routine where they are declared. If analysis indicates that two temporary addresses are never used simultaneously, they may be merged to the same address.

An address knows if it is an address of a byte, of a word, or of a table.

Blocks

Each routine is a block. It may be composed of inner blocks, attached to some instructions.

SixtyPical does not have instructions that map literally to the 6502 branch instructions. Instead, it has an if construct, with two blocks (for the "then" and else parts), and the branch instructions map to conditions for this construct.

The abstract states of the machine at each of the different block exits are merged during analysis. If any register or memory location is treated inconsistently (e.g. updated in one branch of the test, but not the other,) that register cannot subsequently be used without a declaration to the effect that we know what's going on. (This is all a bit fuzzy right now.)

There is also no rts instruction. It is included at the end of a routine, but only when the routine is used as a subroutine. Also, if the routine ends by jsring another routine, it reserves the right to do a tail-call or even a fallthrough.

There are also with instructions, which are associated with an opcode that has a natural symmetrical opcode (e.g. pha, sei). These instructions take a block. The natural symmetrical opcode is inserted at the end of the block.

Loops

Still need to figure this out.

Typical repeat loop looks like:

ldy #0
_loop:
lda #65
sta screen, y
iny
cpy #250
bne _loop

This might be

routine blah {
    ldy# 0
    repeat bne {
        lda# 65
        sta,y screen
        iny
        cpy# 250
    }
}

Note, screen must be a byte table here.

Instruction Support so far

A X indicates unsupported. A ! indicates will-not-support.

Funny syntax indicates use of a special form.

In these, absolute must be a reserved or located address.

X adc #immediate
X adc absolute

X and #immediate
X and absolute

X asl
X asl absolute

  if bcc { block } else { block }

  if bcs { block } else { block }

  if beq { block } else { block }

X bit absolute

  if bmi { block } else { block }

  if bne { block } else { block }

  if bpl { block } else { block }

! brk

  if bvc { block } else { block }

  if bvs { block } else { block }

X clc

X cld

! cli

X clv

  cmp absolute
X cmp #immediate
  
X cpx absolute
X cpx #immediate

X cpy absolute
X cpy #immediate

X dec absolute

X dex

X dey

X eor #immediate
X eor absolute

X inc absolute

X inx

X iny

! jmp

* jsr routine

  lda absolute
X lda #immediate

  ldx absolute
X ldx #immediate

  ldy absolute
X ldy #immediate

X lsr
X lsr absolute

  nop
  
X ora #immediate
X ora absolute

X pha { block }

X php { block }

! pla -- (although note this does change flags)

! plp -- (although note this does change flags -- obviously)

X rol
X rol absolute

X ror
X ror absolute

! rti

! rts

X sbc #immediate
X sbc absolute

X sec

X sed

X sei { block }

  sta absolute
  
  stx absolute
  
  sty absolute

  tax
  
  tay
  
X tsx

  txa

X txs

  tya

TODO

• Parse HEX values like $40A3
• Fuller machine model
• parse support immediate loads, compares
• Addressing modes; rename instructions to match
• Generate code for BEQ

Tests

-> Tests for functionality "Parse SixtyPical program"

-> Functionality "Parse SixtyPical program" is implemented by
-> shell command "bin/sixtypical parse %(test-file)"

-> Tests for functionality "Check SixtyPical program"

-> Functionality "Check SixtyPical program" is implemented by
-> shell command "bin/sixtypical check %(test-file)"

main must be present.

| routine main {
|    nop
| }
= True

| routine frog {
|    nop
| }
? missing 'main' routine

A program may reserve and assign.

| reserve word score
| assign word screen 4000
| routine main {
|    lda screen
|    tax
|    tay
|    cmp score
|    ldx score
|    txa
|    ldy score
|    tya
| }
= True

All declarations (reserves and assigns) must come before any routines.

| routine main {
|    lda score
| }
| reserve word score
? expecting "routine"

All locations used in all routines must be declared first.

| reserve word score
| routine main {
|    lda score
|    cmp screen
| }
? undeclared location

Even in inner blocks.

| reserve word score
| assign word screen 4000
| routine main {
|    lda score
|    cmp screen
|    if beq {
|      lda score
|    } else {
|      lda fnord
|    }
| }
? undeclared location

No duplicate declarations.

| reserve word score
| assign word score 4000
| routine main {
|    nop
| }
? duplicate declaration

-> Tests for functionality "Emit ASM for SixtyPical program"

-> Functionality "Emit ASM for SixtyPical program" is implemented by
-> shell command "bin/sixtypical emit %(test-file)"

| reserve word score
| assign word screen 4000
| routine main {
|    lda screen
|    tax
|    stx score
|    tay
|    sty score
|    cmp score
|    ldx score
|    txa
|    ldy score
|    tya
|    sta screen
| }
= .org 0
= .word $0801
= .org $0801
= .byte $10, $08, $c9, $07, $9e, $32, $30, $36, $31, $00, $00, $00
=   jmp main
= score: .word 0
= .alias screen 4000
= main:
=   lda screen
=   tax
=   stx score
=   tay
=   sty score
=   cmp score
=   ldx score
=   txa
=   ldy score
=   tya
=   sta screen
=   rts